Generally, the prior art describes a multitude of chain links for transport chains in tensioning machines such as film stretching machines. These chains are basically sprocket chains and comprise a plurality of chain links, including so-called outer chain links and inner chain links. The outer chain links are merely chain link plates, while each of the inner chain links includes inner chain link plates interconnected by bearing bushings surrounded by a bearing roller. The outer chain link plates and the inner chain links with their bearing bushings are interconnected by journal pins passing through the respective bearing bushings and through the inner and outer chain link plates. Thus, each bushing surrounds its respective journal pin and the bushing in turn is surrounded by its bearing roller that in operation is contacted by the teeth of a drive sprocket, whereby the bolt functions as a journal.
It is known to insert between the journal pin and the bushing on the one hand, and between the bushing and the bearing roller on the other hand, a friction reducing, elastic bearing material having the required abrasion strength and a smooth surface. Reference is made in this connection to U.S. Pat. No. 4,926,529 (Hosmer et al.), issued on May 22, 1990, for a tenter frame apparatus and method in which the tentering chains comprise elastic, abrasion resistant insert sleeves. Similar disclosures are found in European Patent Publication 0,214,948 (Beeuwsaert), published on Mar. 18, 1987 and in German Patent Publication 1,039,319 (Otto), published on Sep. 18, 1958.
It is a disadvantage of the above mentioned prior art that each inner chain link must be assembled of separate inner link plates, separate bearing bushings and abrasion resistant sleeves. The bearing bushings normally interconnect the respective link plates with each other. Moreover, such chain links require a relatively high machining effort in addition to the just mentioned assembly effort for producing the links and journals for a complete endless tentering chain.
German Patent Publication DE 3,333,938 (Gresens), published on Apr. 11, 1985, discloses a tentering chain for tensioning or film stretching machines in which needle bearings are inserted between the journal pin and a sleeve and between the sleeve and an outer bearing roller resulting in a rather expensive construction.
European Patent Publication 0,119,898 (Gaiffe et al.), published on Sep. 26, 1984, discloses a construction of abrasion resistant, flanged insert bushings (e.g. 7a, 7b) for chain links. Gaiffe et al. also disclose in FIG. 2 integral chain link sections with link plates and bearing bushings that are separated from each other, whereby a proper axial alignment of the chain link bearing bushings is not assured.
It is also known to manufacture the components for the inner chain links as so-called precision casting components by investment casting or lost wax casting to produce these components so that they do not require any machining operation following the casting. However, where machining operations are eliminated substantial disadvantages are involved even with precision cast components, because the on-center spacing between the central rotational axes of the bearing bushings in which the journal pins and bearing sleeves are received, may vary from link to link. As a result, an exact meshing of a drive sprocket wheel with the bearing rollers carried on the bearing bushings of the chain links is not assured. As a result, a substantial wear and tear that reduces the useful life of such chains cannot be avoided. The gear teeth of the drive sprocket are also exposed to substantial wear and tear. Another disadvantage is seen in that even with investment casting the formation of shrinkage cavities is unavoidable so that the cast components must be inspected by an X-ray machine which adds to the end costs of the assembled chain. Another disadvantage is seen in that especially in connection with high speed chain operations, conventional chains do not run quietly and become noisier as they age.